Acetylation of lignocellulosic materials

ABSTRACT

The invention relates to a process for the acetylation of lignocellulosic materials (LM) by (a) bringing the LM into intimate contact with an acetylating agent comprising acetic anhydride as the major component at a temperature from 80°-140° C. and (b) bringing the acetylated LM from step (a) into contact with a heated gas inert under the reaction conditions in a stripper at a temperature above 140° C. and reducing the acetic acid or acetic anhydride content of the acetylated LM produced in step (a) to below 10% by weight by stripping. Products fabricated from LMs so treated are highly dimensionally stable.

This application is a 371 of PCT/GB95/00371, filed Feb. 23, 1995.

The present invention relates to a process for the production ofacetylated lignocellulosic materials wherein the lignocellulosicmaterial is treated with acetic anhydride.

By the expression "lignocellulosic materials" is meant here andthroughout the specification a material in any shape or form such as egshreds, fibres, splinters and shives, and which are derivable from asource comprising wood, sisal, jute, coconut and/or other plant materialand which have optionally been subjected to a pretreatment with anon-acetylating chemical such as eg sodium acetate in order to improvesubsequent acetylation thereof will hereafter be referred to as "LM" forconvenience.

It is well known in the art to improve the physical characteristics ofLMs by treatment with acetic acid and/or acetic anhydride. One of theproblems with systems which use the immersion technique is that they areusually batch processes which need drainage facilities for the liquid inwhich the LM is immersed. A single step process of this type also doesnot ensure acetylation of the lignocellulosic material to the desireddegree nor the removal of all of the unreacted chemicals in the treatedproduct.

It has now been found that the above problems can be mitigated bytreating the LM in two stages, if necessary, in a continuous process.

Accordingly, the present invention is a process for the acetylation oflignocellulosic materials (LM) comprising

a. bringing the LM into intimate contact with an acetylating agentcomprising acetic anhydride as the major component at a temperature from80°-140° C. and

b. bringing the acetylated LM from step (a) into contact with a heatedgas inert under the reaction conditions in a stripper at a temperatureabove 140° C. and reducing the acetic acid or acetic anhydride contentof the acetylated LM produced in step (a) to below 10% by weight bystripping.

In the process, the acetylating agent used in step (a) suitablycomprises at least 50% w/w of acetic anhydride, preferably at least 60%w/w and most preferably at least 70% w/w and typically contains 90%acetic anhydride and 10% acetic acid. This agent may be in the form of aliquid, a vapour or a mixture of the two. Thus the weight ratio ofliquid to vapour in the acetylating agent present in the reactor used instep (a) may be anything within the range from 100% of one to 100% ofthe other, but is suitably from 1:5 to 5:1 and is preferably 1:2. Theacetylating agent used in step (a) is pre-heated to a temperature in theregion from 80°-140° C., preferably from 110°-130° C. using eg an oiljacket or a steam coil.

The LM is contacted with the acetylating agent in a first reactor. Theacetylating reactor is suitably rendered gas tight eg by compactionusing eg a plug screw feeder, at the point of entry into the reactor soas to minimise ingress of air into the acetylation reaction or egress ofacetic anhydride out of this first reactor. This problem of egress ofacetic anhydride can be minimized by operating the first reactor at apressure which is slightly below atmospheric and is also a safetyfeature of the process. The compacted LM is then introduced into thefirst reactor which is suitably a screw-conveyor at the same timeinjecting into said reactor the hot acetylating agent. During this step,the compacted LM is dispersed (in order to loosen the LM strands and tospread it more thinly) using a dispersing device located at the entranceto this first reactor and thereby facilitates intimate contact betweenthe LM and the acetylating agent. In this reactor, the rate of injectionof the hot acetylating agent is suitably such that the ratio of theagent to the LM is suitably in the range from 1:1 to 10:1 by weight,preferably from 3:1 to 7:1, eg 5:1 by weight. During this acetylationreaction, which is exothermic, the reaction temperature is maintainedsubstantially constant which may be achieved by several conventionalmeans, eg by controlling the pressure in the reactor, or, byvapourization of the acetylating chemicals, or, by control of the rateof flow of the acetylating agent into the first reactor using flowcontrol valves and monitoring the rate of flow, or, by controlling thecomposition of the hot acetylating agent injected into the reactor. Itis preferable to achieve this temperature control by the vapourizationmethod whereby the vapour leaving the first reactor is such that it isat its dewpoint. The amount of acetic anhydride in the acetylating agentin the vapour at its dewpoint may be in the range from 30-95% by weight,preferably 70% by weight, which can be filtered to remove any suspendedLM therein and recycled.

The resultant acetylated LM from step (a) suitably contains a controlledamount of liquid which may be up to 110% by weight of the acetylated LMbut is preferably eg about 40% by weight. The amount of liquid in theacetylated LM can be controlled by controlling the flow rate and/or thereaction temperature.

In order to proceed with step (b), the acetylated LM from step (a)containing about 40% by weight of liquid is suitably re-compacted usinga plug screw feed as described previously and dispersed (in order toloosen the LM strands and to spread it more thinly) using a devicesimilar to that used in the acetylation reactor and in order to create agas tight seal in the stripper. However, the same effect may be achievedusing eg a rotary valve. The gas inert under the reaction conditions issuitably nitrogen, argon, carbon dioxide or the like which is heatedbefore it is introduced at a point after the acetylated LM is dispersedso as to entrain said dispersed, acetylated LM and to transport it tothe stripper. The hot inert gas is at a temperature above 140° C.,suitably from 140°-220° C., preferably from 185°-195° C. and the secondreactor is suitably operated at a pressure of 100-150 KPa. In thestripper, the acetylated LM is brought into contact with the hot inertgas. The function of this step (b) is to reduce the content ofacetylating medium remaining in the acetylated LM emerging from step (a)ie by stripping. Thus, vapours removed from this reactor can berecirculated to the hot acetylating agent used in the first reactor.

The duration of the acetylation reaction of the LM in step (a) will bedetermined by the degree of acetylation desired and the nature andamount of the acetylatable material present in the LM. Thus the degreeof acetylation is suitably such that the acetylated LM achieves a weightgain of at least 2%, preferably at least 5% and more preferably from5-25% by weight.

The stripped, acetylated LM product from step (b) can be furtherprocessed in a hydrolysis chamber in order to remove or at leastminimize the odor of the chemicals in the treated LM. For instance, thestripped product which is usually at an elevated temperature of about130°-160° C. may be dispersed (in order to loosen the LM strands and tospread it more thinly) and entrained in a current of steam optionallywith one or more other entraining gases. It is preferable to usesuperheated steam and in this instance, any residual unreacted aceticanhydride remaining adsorbed or occluded in the acetylated LM ishydrolyzed to acetic acid and removed overhead as a mixture of steam andacetic acid from the hydrolysis chamber. This mixed vapour may berecirculated to the superheated steam being introduced into thehydrolysis chamber. The effect of this step is that it significantlyreduces the acetic acid or anhydride odor of the treated LM product byremoving substantially all of the acetic anhydride therefrom and alsomost of the acetic acid therein. The amount of acetic acid left behindin the acetylated product is suitably less than 0.5% w/w.

The acetylated LM after removal of any residual anhydride usingsuperheated steam and emerging from the hydrolysis chamber is usually ata temperature of about 150° C. This product can be sent either directlyto an adjoining plant where said product can be formed into the desiredshapes eg boards, or, can be subjected to a further humidifying and acooling step in order to bag the material for storage and distribution.This may be achieved by passing a current of steam and air over thetreated LM emerging from the hydrolysis chamber so as to cool thetreated LM to about 40° C. and to humidify it.

The process of the present invention is particularly suitable foracetylating fibres of LM, especially wood fibres, which may vary indimensions eg from 0.03 mm to 4 mm. Furthermore, the process of thepresent invention can be operated batchwise or continuously, an optionnot available in the conventional processes used hitherto.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is further illustrated with reference to theaccompanying schematic flow diagram and the associated descriptionbelow.

Raw LM eg the fibre is fed via a hopper (1) to a plug-screw feeder (2)where the fibre is compacted to reduce the permeability of the fibre togas flow and to render the acetylating reactor gas tight. The plug-screwfeeder (2) discharges the compacted fibre to the first reactor (3) whichis purged with nitrogen before the injection of a pre-heated acetylatingagent which is a mixture of acetic acid (about 10% by wt) and aceticanhydride (about 90% by wt) both in liquid and in vapour form alonglines (4) and (5) respectively into said reactor (3). The compactedfibre is dispersed with a dispersing device (not shown) located at theentrance of the compacted material into reactor (3) as the fibre isbrought into contact with the acetylating agent. Reactor (3) ismaintained at a pressure slightly below atmospheric in order to preventback flow of the acetylating agent into the plug-screw feeder or intolines (4) and (5). Reactor (3) is maintained at a temperature of about120° C. by means of steam heating. The acetylation of the fibre is anexothermic reaction and the reaction temperature is maintained by thevapourization of the liquid components of the acetylating agent therein.Chemical vapour at its dewpoint, containing about 70% by weight aceticanhydride emerges overhead from this reactor and is filtered beforebeing recovered and recycled. The amount of liquid in the acetylatedfibre emerging from the base of reactor (3) is controlled at about 40%by weight and this fibre is re-compacted in a further plug-screw feeder(6) before being dispersed again in a dispersing device (7) andsubjected to a further treatment with the inert gas, eg nitrogen heatedto about 190° C. The acetylated fibre so treated is entrained in theheated gas stream emerging from the base thereof and transported to astripper (8) which is a steam jacketed circulation stripper where thechemicals adsorbed or occluded in the acetylated fibre are evaporated.The overheads from the circulation stripper (8) entraining theacetylated fibre are fed into a circulation cyclone (9) where hotacetylated fibre is recovered from the base thereof and is fed through acell feeder (not shown) into line (10). In line (10) the acetylatedfibre is dispersed in and entrained by a mixture of superheated steamand some acetic acid vapour (from partly recycled streams) and is thenfed into a steam stripper (11). In (11), any residual acetic anhydridein the acetylated fibre is hydrolyzed to acetic acid and the acetic acidis stripped out. The overheads from the steam stripper (11), which areat a temperature of about 150° C., are fed into a steam stripper cyclone(12) where the acetylated fibre is separated from the vapours andrecovered from the base thereof whereas the acidic vapours are recoveredoverhead therefrom to be processed and recycled. The acetylated fibressubstantially free of all free acids and anhydrides (and hence the odorsof these components) recovered from the base of (12) can either be sentto an adjacent board manufacturing facility (not shown) directly or canbe humidified and cooled for the purposes of bagging and storage (notshown).

The present invention is further illustrated with reference to thefollowing Example:

EXAMPLE

A mixture of fibres of spruce and pine (1.5 g) was acetylated for 10minutes with a flow of saturated acetic anhydride vapour (containing 5%w/w of acetic acid) at its dewpoint at atmospheric pressure. The totalacetyl content achieved by this method was 15.5% and the residual acidon the acetylated fibre as defined below was 74.6% w/w. The acetylatedfibre was then stripped using varying nitrogen flows as shown in theTables below. The amount of acetylating agent left on the fibre atvarious stages during this stripping step is expressed as acid (methodused to determine level is water based, therefore all anhydride ishydrolyzed to acetic acid). The results are shown in Tables 1 and 2below: ##EQU1##

                  TABLE 1                                                         ______________________________________                                        N.sub.2 flow = 2.4 1/minute through 1.5 g of acetylated fibre at              190° C.                                                                                  Residual Acid                                               Stripping Time in Minutes                                                                       (%)                                                         ______________________________________                                        0                 74.6                                                        1                 64.6                                                        3                 10.7                                                        5                  2.9                                                        10                 0.3                                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        N.sub.2 flow = 5.4 1/minute through 1.5 g of acetylated fibre at              190° C.                                                                                  Residual Acid                                               Stripping Time in Minutes                                                                       (%)                                                         ______________________________________                                        0                 74.6                                                        1                 48.1                                                        2                 17.2                                                        3                  5.7                                                        5                  0.6                                                        ______________________________________                                    

We claim:
 1. A process for the acetylation of lignocellulosic materials(LM) comprising a. bringing the LM into intimate contact in a reactorwith an acetylating agent comprising acetic anhydride as the majorcomponent and acetic acid as a minor component at a temperature from80°-140° C. and b. bringing the acetylated LM from step (a) into contactwith a heated gas inert under the reaction conditions in a stripper at atemperature above 140° C. and reducing the acetic acid or aceticanhydride content of the acetylated LM produced in step (a) to below 10%by weight by stripping.
 2. A process for the acetylation oflignocellulosic materials (LM) comprisinga. bringing the LM intointimate contact in a reactor with an acetylating agent comprisingacetic anhydride as the major component at a temperature from 80°-140°C. and b. bringing the acetylated LM from step (a) into contact with aheated gas inert under the reaction conditions in a stripper at atemperature above 140° C. and reducing the acetylation content of theacetylated LM produced in step (a) to below 10% by weight by stripping.3. A process according to claim 1 or claim 2 wherein the acetylatingagent used in step (a) comprises at least 20% w/w of acetic anhydride.4. A process according to claim 1 or claim 2 wherein the acetylatingagent used in step (a) comprises 90% w/w acetic anhydride and 10% w/wacetic acid.
 5. A process according to claim 1 or claim 2 wherein theacetylating agent is in the form of a liquid, a vapour or a mixture ofthe two.
 6. A process according to claim 2 or claim 5 wherein the weightratio of liquid to vapour in the acetylating agent present in thereactor of step (a) is from 1:5 to 5:1.
 7. A process according to claim1 or claim 2 wherein the acetylating agent used in step (a) ispre-heated to a temperature in the region from 80°-140° C.
 8. A processaccording to claim 1 or claim 2 wherein the reactor used in step (a) ismaintained at a pressure below atmospheric during the acetylationreaction.
 9. A process according to claim 1 or claim 2 wherein the rateof injection of the hot acetylating agent into the reactor of step (a)is such that the ratio of the acetylating agent to the LM is in therange from 1:1 to 10:1 by weight.
 10. A process according to claim 1 orclaim 2 wherein the acetylated LM from step (a) contains a controlledamount of liquid which may be up to 110% by weight of the acetylated LM.11. A process according to claim 1 or claim 2 wherein the acetylated LMfrom step (a) is entrained in the gas inert under the reactionconditions which is heated before it comes into contact with theacetylated LM and transported to step (b).
 12. A process according toclaim 1 or claim 2 wherein the inert gas comprises nitrogen, argonand/or carbon dioxide and is at a temperature above 140° C.
 13. Aprocess according to claim 1 or claim 2 wherein the degree ofacetylation achieved is such that the acetylated LM achieves a weightgain of at least 2%.
 14. A process according to claim 1 or claim 2wherein the stripped, acetylated LM product from step (b) is furtherprocessed in a hydrolysis chamber by contact with a hot gas comprisingsteam whereby any residual unreacted acetic anhydride remaining adsorbedor occluded in the acetylated LM is hydrolysed to acetic acid andremoved overhead as a mixture of steam and acetic acid from thehydrolysis chamber so that the acetic acid in the acetylated product isless than 0.5% w/w.